1. Technical Field
The present invention relates to a pulse generating circuit for generating pulses appropriate for UWB (ultra wide band) communication, and a UWB communication system.
2. Related Art
A UWB communication system is a communication system which provides high-speed and large-capacity data communication using an extremely wide frequency band. For generating wide-band signals, the UWB communication system employs method utilizing extremely short-period pulses as well as spectrum diffusion method and orthogonal frequency division multiplexing (OFDM) method used in related art. The system using the extremely short pulses is particularly called impulse radio (IR) system communication. The IR system can achieve modulation and demodulation not by the related-art modulation method but only by time base operation. Thus, simplification of circuits and reduction of power consumption are expected for this system (see U.S. Pat. No. 6,421,389, US Patent Application Publication No. 2003/0108133A1, and US Patent Application Publication No. 2001/0033576).
The pulse waveform used in the IR system is now briefly explained. Pulse waves having a pulse width PD and a cycle TP shown in FIG. 16A are well known. The frequency spectrum of these pulse waves is sinc function whose envelope has first zero point when BW=1/PD as shown in FIG. 16B.
This pulse waveform whose spectrum extends to BW from direct current is difficult to use, and such a pulse waveform whose spectrum center is located at a high position in the frequency as shown in FIG. 16D is preferable. This pulse type of waveform is the pulse waveform shown in FIG. 16C whose frequency spectrum has been shifted to a higher position by multiplying pulses shown in FIG. 16A by pulse waves having frequency f0=1/2Pw. Each section of the pulse width PD contains several pulses having a width Pw (Pw=1/(2f0)) as half of the carrier wave cycle. However, this waveform includes direct current (DC) components indicated by an alternate long and short dash line 1601 in FIG. 16C, and does not accurately have ideal spectrum shown in FIG. 16D.
FIG. 16E shows a waveform having this ideal spectrum. This waveform has pulses shown in FIG. 16A multiplied by sine waves at the carrier frequency f0. FIG. 16F shows a waveform having pulses shown in FIG. 16A multiplied by rectangular waves at the carrier frequency f0, and is easily generated in digital circuits. Since digital circuits produce narrow pulse width, the generated waveform is not angular as shown in FIG. 16F but generally becomes a waveform shown in FIG. 16E. Other pulse waves different from those shown in the figure but ideal for the UWB communication have been currently proposed, and many of them are used due to easiness of generation methods.
Related Art 1
FIG. 17A shows an example of a circuit in related art which generates pulses shown in FIG. 16C (see A CMOS IMPULSE RADIO ULTRA-WIDEBAND TRANCEIVER FOR 1 Mb/s DATA COMMUNICATIONS AND ±2.5 cm RANGE FINDINGS, T. Terada et. al, 2005 Symposium on VLSI Circuits Digest of Technical Papers, pp. 30-33). Two inverters 1701 and 1702 and an NOR circuit 1703 constitute three ring generating circuits when the other input Ci of the NOR 1703 is false (L: low level). More specifically, as shown in the timing chart in FIG. 17B, pulses are generated only while Ci is at L level, and changes of output NR of the NOR 1703 and outputs N1 and N2 of the inverters 1701 and 1702 are transmitted with delay time td for each.
For simplifying the explanation, it is assumed herein that the rising time and falling time of the NOR 1703 and the inverters 1701 and 1702 are all the same. Thus, the pulse width (Pw in FIG. 16C) of the pulses generated from this circuit is 3 td. In this case, the shortest possible pulse width generated by this circuit is three times longer than the delay time of the elements constituting the circuit, which is the smallest pulse width of the pulses generated by this circuit.
Related Art 2
According to the UWB communication, the pulses generated by this method are used not only by a transmitting device but also by a receiving device as template pulses for calculating correlation with receiving signals. The receiving device processes differential signals in many cases, and often requires two types of signals whose phases are reversed as shown in FIG. 16G. Differential pulse signals are effectively used by the transmitting device as well at the time of actuation of balanced antenna or for other purpose. The receiving circuit further requires I and Q orthogonally crossing signals whose phases are different from each other by 90 degrees in many cases.
A Low-Power Template Generator for Coherent Impulse-Radio Ultra Wide-Band Receivers, Jose Luis et. al, Proceedings IEEE ICUWB, 2006 pp 97-102, discloses a circuit for generating balanced pulses. According to this circuit, several differential circuits are connected in line to generate pulse waves having a pulse width corresponding to delay of one delay circuit by using logic circuits. This reference also describes that pulses can be generated at both rising and falling of signals inputted to the delay circuits so as to achieve power consumption reduction, and that the I and Q signals can be generated by using every other circuit of the connected delay circuits.
According to the related-art technologies, both Di and XDi are always generated due to complementary structure of the delay circuits. Thus, the I and Q signals can also be easily generated. In the method which complementarily uses P channel MOS transistors and N channel MOS transistors to obtain differential signals, however, unbalanced signals are generated when the constants of both the P and N channel MOS transistors are unbalanced. When signals contain unbalanced components, output errors increase particularly in such a case where the receiving device has a correlation unit.
Moreover, according to the above disclosures, pulses are generated at both the rising and falling edges of the generation starting signals so as to achieve power savings. However, the polarities of the pulses generated at the rising timing and the pulses generated at the falling timing are reversed, which imposes severe limitation to modulating operation and generation timing.